The present invention relates to improvements in an exposure condition determining means of a photographic printing apparatus in which printing of variable magnification rate is conducted by zooming so that a print can be obtained from a negative.
Referring to FIG. 5 in which the side view of the composition of a photographic printing apparatus is illustrated, the prior art will be explained. An image of the negative film 1 located on a predetermined position on the negative mask 1a, is irradiated with diffused light having a predetermined luminance, wherein the diffused light is coming from the light source 9 through the heat absorbing filter 8, the B G R [Blue.Green.Red] cut sheet filter 7 for color separation, and the mirror box 15. The image of the above-described negative film 1 is projected by the zoom lens 3 and the projected image is focused on the emulsion coated surface of the photographic paper 6 via the reflecting mirror 5. In order to make a projected image of a predetermined magnification rate from the above-described negative film 1, the lens element of the above-described zoom lens 3 is moved in the direction of the optical axis so that the focal length can be changed. When the above-described photographic paper 6 is exposed, the appropriate exposure is determined by the most appropriate combination of the aperture size of the diaphragm 2 in the above-described zoom lens and the opening time period of the shutter 4 located at a predetermined position on the optical path. When an image is magnified to a size different from the standard size by zooming or an image is magnified to an optional size and the magnified image is trimmed by a variable mask provided to the negative film 1, the exposure is determined in such a manner that: the opening time period of the above-described shutter 4 is changed according to the magnification rate and the diaphragm 2 is manually adjusted in order to select the appropriate diaphragm aperture size so that the exposure amount can be set to the appropriate value which is the same as the exposure to the standard print size. The measurement of image illuminance at the position close to the focussing surface and the trial printing are conducted in order to determine the appropriate diaphragm aperture. Then the exposure conditions are also determined.
Generally speaking, when projected images of a plurality of magnification rates are made from the full size of negative film 1 or the trimmed negative image, the focal length of the above-described zoo lens 3 is changed. As the aperture diameter D of a lens is constant, according to the equation of F.sub.NO =f/D, F-number (F-number will be called F hereafter in this specification.) which indicates the brightness of a lens, is varied with the change of the focal length. For that reason, the exposure of the projected image must be changed when the images of various magnification rates are made from the negative film 1 having a constant luminance through the above-described zoom lens 3. For example, assuming that: the shutter opening time period is constant; the above-described zoom lens 3 is used; the negative film 1 of 35 mm size is applied to the printer; and the diaphragm aperture size with which the projected image of the economical print size E is exposed on the above-described printing paper 6 is the standard value to get the most appropriate exposure amount. When the print of 8.times.10 inch size or 10.times.12 inch size is made, the focal length of the above-described zoom lens must be changed. Accordingly, F.sub.NO of the zoom lens 3 is necessarily changed. Therefore, the diaphragm aperture size must be changed so that the standard exposure amount to the above-described E print size can be given to the print. When the negative size is the half size, which is a half of 35 mm size, is printed to E size ,the diaphragm aperture size must be changed in the same way as described above so that the exposure amount which is the same as the standard exposure can be given to the print. The conventional method to change the magnification rate is as follows: the diaphragm aperture size is changed manually or the shutter opening time period is changed while trial printing is conducted; and it is judged whether the exposure amount is the same as the most appropriate image exposure amount or not. This method is not efficient in the case of a high speed photographic printing apparatus in which not less than 30 sheets per minute of prints can be processed, wherein the apparatus must be stopped in order to set up each time the magnification rate is changed.
Exposure amount is determined by a worker according to his experience, so that the exposure amount varies from print to print when the magnification is changed, which deteriorates the quality of prints.